utf8.[ch] et al: use char32_t and char16_t instead of int, int32_t, int16_t

rework C11 utf8.[ch] to use char32_t instead of uint32_t when referring
to unicode chars, to make things more expressive.

[
 @zonque:
  * rebased to current master
  * use AC_CHECK_DECLS to detect availibility of char{16,32}_t
  * make utf8_encoded_to_unichar() return int
]

src/basic/missing.h

@@ -36,6 +36,7 @@
 #include <stdlib.h>
 #include <sys/resource.h>
 #include <sys/syscall.h>
+#include <uchar.h>
 #include <unistd.h>
 
 #ifdef HAVE_AUDIT
@@ -1159,4 +1160,13 @@ static inline key_serial_t request_key(const char *type, const char *description
 
 #ifndef IF_OPER_UP
 #define IF_OPER_UP 6
+
+#ifndef HAVE_DECL_CHAR32_T
+#define char32_t uint32_t
+#endif
+
+#ifndef HAVE_DECL_CHAR16_T
+#define char16_t uint16_t
+#endif
+
 #endif